Passive phase control in an external cavity laser

ABSTRACT

An external-cavity laser module includes a package defining an enclosure, the package including a base having a surface internal to the enclosure, a thermoelectric cooler within the enclosure, the thermoelectric cooler including an upper carrier plate and a lower carrier plate, the lower carrier plate being placed on the internal surface of the base and the thermoelectric cooler (TEC) being configured to stabilize the temperature of the upper carrier plate at a substantially constant temperature. The laser module further includes a laser assembly housed within the enclosure, including a gain medium for emitting an optical beam into the external cavity and an end mirror. Variations of the environmental temperature with respect to the thermally stabilized temperature cause mechanical deformations of the TEC upper carrier plate that is in thermal coupling with the laser assembly. The mechanical deformations in turn induce variations in the optical path length of the laser cavity. Thermal bridge of the gain medium to the environmental temperature is achieved by the use of a thermal bridge element for conducting heat either from or to the gain medium.

CROSS REFERENCE TO RELATED APPLICATION

This application is a national phase application based onPCT/EP2005/013060, filed Dec. 6, 2005, the content of which isincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an external-cavity laser and in particular toan external cavity tuneable laser that is especially adapted as opticaltransmitter for wavelength-division multiplexed optical communicationnetworks.

2. Description of the Related Art

The use of lasers as tuneable light source can greatly improve thereconfigurability of wavelength-division multiplexed (WDM) systems or ofthe newly evolved dense WDM (DWDM) systems. For example, differentchannels can be assigned to a node by simply tuning the wavelength.Also, tuneable lasers can be used to form virtual private networks basedon wavelength routing, i.e., photonic networks.

Different approaches can be used to provide tuneable lasers, such asdistributed Bragg reflector lasers, VCSEL lasers with a mobile topmirror, or external-cavity diode lasers. External-cavity tuneable lasersoffer several advantages, such as high output power, wide tuning range,good side mode suppression and narrow linewidth. Various laser tuningmechanisms have been developed to provide external-cavity wavelengthselection, such as mechanically adjustable or electrically activatedchannel selector elements.

U.S. Pat. No. 6,526,071 describes an external-cavity tuneable laser thatcan be employed in telecom applications to generate the centrewavelengths for any channel on the International TelecommunicationsUnion (ITU) grid. The disclosed tuneable laser includes a gain medium, agrid generator and a channel selector, both grid generator and channelselector being located in the optical path of the beam. The gridgenerator selects periodic longitudinal modes of the cavity at intervalscorresponding to the channel spacing and rejects neighbouring modes. Thechannel selector selects a channel within the wavelength grid andrejects other channels. The grid generator is dimensioned to have a freespectral range (FSR) corresponding to the spacing between gridlines of aselected wavelength grid (an ITU grid) and the channel selector isdimensioned to have a FSR broader than that of the grid generator whichis itself broader than the FSR of the cavity.

Typically, the grid generator is a Fabry-Perot etalon defining aplurality of transmission peaks (also referred to as passbands) definingperiodic longitudinal modes. To select a periodic longitudinal mode(i.e., a lasing channel on the ITU grid), several channel selectingmechanisms have been considered, including rotating a diffractiongrating, mechanically translating a wedge-shaped etalon, or varying thevoltage or current supplied to an electro-optically controlled element.

J. De Merlier et al. in “Full C-Band External Cavity Wavelength TunableLaser Using a Liquid-Crystal-Based Tunable Mirror”, published in IEEEPhotonics technology Letters, vol. 17, No. 3 (2005), pages 681-683,disclose an external cavity tuneable laser containing a fixed etalonwith a FSR of 50 GHz and a liquid crystal (LC) based tuneable mirror.The tuneable mirror is an optical resonator that works in reflection,exhibiting one reflection peak over a wide wavelength range which can betuned over the whole C-band by adjusting the amplitude of the ac voltagesignal. The laser consists of a chip containing a gain and a phasesection. The integration of the phase control on the chip avoids theneed for mechanical tuning of the cavity length.

An external cavity tuneable laser with an etalon as grid generator andan LC-based tuneable mirror is described in WO patent application No.2005/041371.

In order to accommodate increasing optical communication traffic, DWDMsystems with channel spacing of 50 GHz and even of 25 GHz have beenrecently developed. As DWDM uses narrower channel spacing, wavelength(frequency) accuracy of transmitter lasers over the entire tuning (e.g.,the C-band) and operating temperature range has become an importantissue. DWDM systems with 50 GHz channel spacing typically require anaccuracy of ±2.5 GHz about the lasing frequency, whereas systems with 25GHz generally require a frequency accuracy of ±1.25 GHz.

As tuneable elements are configured for narrower channel separation,decreasing component tolerances and thermal fluctuations becomeincreasingly important. Spatial misalignments of optical components ofthe laser device may arise from temperature variations due to expansionsand contractions associated to the various components, which will reducewavelength stability and generally reduce the performance of the laser.The laser response needs to be stabilised across a relatively widetemperature range, typically ranging from −5° C. to 70° C. To ensurethermal stability, many telecommunication laser devices are mounted on acommon platform, which exhibits high thermal conductivity and is subjectto the thermal control of one or more thermo-electric coolers (TECs).Temperature control allows for maintenance of thermal alignment of theoptical components.

In an external cavity laser, a resonant external cavity is formed withoptical path length L_(opt) between a first mirror, typically thereflective rear surface of the gain medium, and a second mirror, the endmirror. The free spectral range (FSR) of the laser cavity, i.e., thespacing between the cavity modes, depends on the optical path length,owing to the relation

$\begin{matrix}{\left( {F\; S\; R} \right) = \frac{c_{0}}{2L_{opt}}} & (1)\end{matrix}$wherein c₀ is the speed of light in vacuo.

The optical path length of an external cavity laser is a sum of theproducts of indices of refraction and optical thicknesses of the variouselements or components present in the optical path across the externalcavity, including the air present within the cavity. Thus, the opticalpath length of the laser cavity can be shown asL _(opt)=Σ_(i) n _(i) L _(i)  (2)where n_(i) (i=1, . . . , m) is the refractive index of the mediumfilling the i^(th)-optical element (component) that the light encountersin the cavity and of the cavity itself (i.e., the free space, n_(FS)≈1),while L_(i) is the thickness of the i^(th)-element and the physicallength the light travels in free space (i.e., the free-space physicallength). The external cavity can be thought of as an optical resonatorcomposed of two confronting and reflective, generally parallel, surfacesseparated by a length, which is defined as the physical length of thecavity, L₀. In general, L_(opt)≧L₀.

U.S. Pat. No. 6,658,031 discloses a laser apparatus that uses an activethermal adjustment of a laser cavity reflective element to minimiselosses and provide wavelength stability. A compensating member iscoupled to a reflector and configured to thermally position the onereflector with respect to the other reflector in order to maintain anoptical path length that does not vary with temperature (except duringactive thermal control of the compensating member). The thermalpositioning may be carried out by a thermoelectric controlleroperatively coupled to the compensating member and configured tothermally adjust the compensating member by heating or cooling thereof.

In U.S. Pat. No. 6,724,797, an external-cavity laser device isdisclosed, wherein selective thermal control is applied to opticalcomponents having a high susceptibility to thermal misalignments. Thegain medium and the optical output assembly, which are temperaturesensitive components, are mounted on a thermally conductive substrate. ATEC is coupled to the substrate to allow for the gain medium and theoutput assembly to be thermally controlled independently from the endmirror and other components of the external cavity laser. Components ofthe external cavity, which are thermally isolated from the thermallyconductive substrate, may comprise a channel selector and a tuningassembly.

From Eq. (2) it can be seen that L_(opt) may be adjusted by physicaladjustment of the spacing between the two end mirrors defining theexternal cavity and/or by adjusting the refractive index of the materialpresent in the external cavity. Semiconductor gain media such as InGaAsand InGaAsP have generally high refraction indices that exhibitrelatively large variations with temperature and therefore cansignificantly contribute to the overall external cavity optical pathlength.

U.S. Pat. No. 6,763,047 describes an external cavity laser apparatusthat uses an active thermal adjustment of the external cavity tominimise losses and provide wavelength stability. The apparatus of thecited patent includes a thermally conductive platform, a gain medium andan end mirror thermally coupled to the platform and a thermoelectriccontroller thermally coupled to the platform and configured to cause theplatform to expand and contract in response to a temperature change ofthe platform, thereby adjusting the optical path length of the cavity.Heating or cooling of the platform by the thermoelectric controllerprovides temperature control of the gain medium refractive index viathermal conduction with gain medium and/or thermal expansion orcontraction of the platform to control the spacing between the endmirrors. A control element is operatively coupled to the thermoelectriccontroller to provide control instructions regarding heating or coolingof the platform, and hence of the gain medium.

In the field of integrated circuits, different techniques and structureshave been proposed to remove the heat generated by the operation ofsemiconductor devices in order to maintain the temperature of thedevices within a predetermined range.

U.S. Pat. No. 4,442,450 describes an electronic semiconductor packagehaving a support substrate, an integrated circuit semiconductor devicemounted on said substrate, a cover mounted on said substrate disposedover said device and thermal bridge for conducting heat from said deviceto said cover. The thermal bridge comprises a relatively thick metalsheet provided with grooves and cuts that make the thermal bridgebendable, said metal sheet being overlaid by a spring element toselectively urge part of the bridge into contact with the device.

In U.S. Pat. No. 4,479,140, a thermal bridge element is used in asemiconductor package to conduct heat from a semiconductor devicemounted on a substrate to a cold plate or cap in close proximity to thedevice.

SUMMARY OF THE INVENTION

The present invention relates to an external cavity laser assemblycomprising a gain medium, which is in thermal coupling to athermoelectric cooler (TEC). The laser assembly preferably comprises anend mirror, which is preferably in thermal coupling to a thermoelectriccooler.

The gain medium is preferably a semiconductor gain medium, such as asemiconductor laser chip. Due to environmental thermal fluctuations andto heating generated during operation, semiconductor gain media undergothermal fluctuations that in turn induce variations of the refractiveindex with consequent changes of the optical path length of the laserexternal cavity. In order to improve temperature stability, the gainmedium is thermally coupled to a thermoelectric cooler (TEC) thatprovides the gain medium with thermal control. The TEC comprises asurface in thermal coupling with the gain medium so that excessiveheating of the gain medium can be dissipated through the TEC, which isoperatively arranged so as to maintain said TEC surface at asubstantially constant temperature. Within this context, a substantiallyconstant temperature refers to a temperature stabilised within a narrowrange with respect to its mean value, e.g., 25° C.±0.2° C. or 22°C.±0.5° C. As it will become clearer from the following discussion,thermal coupling between the gain medium and the TEC means that a heatflow path with a defined thermal resistance exists between the gainmedium and the TEC thermally stabilised surface.

The TEC which the gain medium is thermally coupled to includespreferably a Peltier cell. A Peltier cell is a well known semiconductorjunction device that can produce heat or cold on one of it surfacesdepending on the direction of the current applied to it andindependently of the environmental temperature. The change oftemperature is achieved by the use of the well-known Peltier effect, inwhich a lower temperature is created on one side of a semiconductorjunction array or layer, and an elevated temperature on the oppositeside. This essentially leads to a transfer of heat to or from a firstsurface from or to an underlying substrate or structure. The temperatureof the first surface can thereby be changed, either increased ordecreased, by a current applied to the device, in the appropriatedirection. To increase the effect, it is necessary to increase thecurrent density. Thus, by the mere exertion of an electrical current,the temperature of the first surface can adaptively be adjusted andchanged. This effect is reversible. If the direction of current ischanged, the original cooling side will become the heating side and theheating side becomes the cooling side.

The TEC including a Peltier cell can have a standard constructioncomprising two ceramic carrier plates with a series of P- and N-dopedsemiconductor materials (i.e., the Peltier cell), typically consistingof several hundred PN couples of bismuth-telluride semiconductormaterial, sandwiched between them. Typically, the two carrier plates areof thermally conductive ceramic, such as AlN or Al₂O₃.

For thermal stabilisation, operation of the Peltier cell is set so thatthe temperature of one of its sides, i.e., its first surface, ismaintained at the same temperature within a given narrow temperaturerange, e.g., 25° C.±0.1° C. Considering a typical construction of a TECincluding a Peltier cell, this means that one of the TEC carrier platesis thermally stabilised, said carrier plate including a surface,hereafter referred to as the thermal interface or the thermallystabilised surface, which the gain medium of the external cavity laser,and preferably also the other optical components within the lasercavity, is in thermal coupling to. The TEC carrier plate maintained at asubstantially constant temperature will be referred to as the “upper”carrier plate with respect to the Peltier cell, whereas the carrierplate at about the environmental temperature will be referred to as the“lower” plate, reflecting a typical orientation of a thermallycontrolled cavity laser.

In one of its aspects, the present invention relates to an externalcavity laser module comprising a package defining an enclosure andincluding a base and a laser assembly placed within the enclosure, saidlaser assembly comprising a gain medium placed in thermal coupling tothe thermally stabilised surface of a TEC.

According to a preferred embodiment, the laser assembly comprising thegain medium and the end mirror is placed on a common thermallyconductive platform, i.e., an optical bench, which is in thermal contactwith the thermally stabilised surface of a TEC.

Applicant has observed that environmental temperature variations mayinduce mechanical deformations of the thermally stabilised surface ofthe TEC, i.e., of the thermally stabilised TEC carrier plate. Saiddeformations are due to a temperature difference between the surfacemaintained at a substantially constant temperature and the lower surfaceunderlying the semiconductor array (i.e., the lower carrier plate),which is at the environmental temperature. Depending also on the thermalexpansion coefficient of the material making the two carrier plates ofthe TEC, the bigger the difference between the substantially constanttemperature and the environmental temperature, the larger the mechanicaldeformation.

In the preferred embodiments of the present invention, the gain mediumis placed on a thermally conductive platform, which is in thermalcontact with the thermally stabilised surface of the TEC, e.g., bondedto the TEC upper carrier plate. In case of a thermally conductiveplatform mounting the gain medium (either directly or through asubmount), mechanical deformations of the thermally-stabilised carrierplate can be transmitted to the thermally conductive platform. Themagnitude of the transmitted compressive or tensile strain depends onthe Young's modulus and on the thickness of the thermally conductiveplatform. A highly rigid platform (e.g., a CVD-diamond base plate withYoung's modulus of about 1050 GPa or a SiC base plate with Young'smodulus of about 470 GPa) would not be significantly affected bydeformations in the underlying layers. Similarly, a thermally conductiveplatform having a large thickness, e.g., larger than about 1.5 mm, wouldto a large extent dampen down the deformations occurring in theunderlying carrier plate. However, thermally conductive platformssuitable as optical benches for laser assemblies made of a materialhaving very large Young's modulus, e.g., CVD-diamond or SiC, are alsovery expensive. On the other hand, optical benches having relativelylarge thickness often do not match the package constraints.

Mechanical deformations of the TEC thermal interface (or of thethermally conductive platform on the TEC) in turn induce variations inthe optical path length of the laser cavity, mostly due to variations ofthe free-space physical length. For instance, a variation of theenvironmental temperature from 25° C. to 70° C. may lead to a reductionof the optical path length from a few tenths of micron to more than 2μm, depending on several factors, such as the surface area of the TECupper carrier plate and the thickness of the thermally conductiveplatform.

In external-cavity tuneable lasers for WDM systems, variations of thecavity optical path length cause an offset of the cavity mode from thecentre of the (selected) etalon peak. Such an optical misalignment ofthe cavity modes introduces optical losses that lead to a drop in theoutput power at the selected lasing channel, which can be unacceptablewhen a stable output at selectable wavelengths during laser operation isrequired. The optical misalignment involves also a frequency shift ofthe output frequency (i.e., the lasing wavelength), such a shift rangingfor instance from 100 MHz to 1 GHz.

Although a possible solution resides in using an electronicallycontrolled component coupled to an optical element of the laser cavity(e.g., a piezoelectric actuator that adjust the position of the endmirror), in order compensate for the optical path length changes, activeadjustments would introduce complexity in the laser package, therebyincreasing the manufacturing costs.

Centering of the lasing channel could be attained by monitoring thelaser output power and by making adjustments of the injection current ofthe gain medium, until the power is maximised. However, the Applicanthas observed that when deformations of the TEC upper plate are morepronounced because of a relatively large temperature gradient betweenthe stabilised temperature and the environmental temperature (e.g., morethan 20° C.; within the operative temperature range, gradients can be upto 45-50° C.), the adjustments of the injection current necessary tocompensate the optical effect of the deformations are relatively large,e.g., up to about 60-70 mA. It follows that the variations of theoptical power needed to restore the alignment condition are significantand can adversely affect the stability of the laser output byintroducing relatively large fluctuations, e.g., of 1 dB, in the outputoptical power.

The Applicant has therefore understood the need of reducing the effectof mechanical deformations so that adjustments of the injection currentof the gain medium are either unnecessary or at least can occur within anarrow range of values, preferably within about 10 mA and morepreferably within 5 mA. In this way, the stability of the lasing signalis not adversely affected.

FIG. 1 schematically illustrates a typical configuration of a thermallycontrolled external cavity laser. The external cavity laser 1 includes asemiconductor gain chip 2 as gain medium emitting a light beam and anend mirror 3, the gain medium and preferably the end mirror beingthermally coupled to a thermoelectric cooler 4. The gain medium 2 isplaced on a thermally conductive submount 9, which is placed on athermally conductive platform or optical bench 10. The end mirror 3 isalso placed on the optical bench 10. The thermoelectric cooler (TEC) 4includes a Peltier cell 7, an upper carrier plate 5 including a thermalinterface 6, the temperature of said thermal interface being maintainedat a given temperature, T₁, and a lower carrier plate 8 placed at theenvironmental temperature, T_(env). “Upper” and “lower” plates, 5 and 8,with respect to the Peltier cell 7 are referred to a typical orientationof a thermally controlled cavity laser, i.e., the one shown in FIG. 1.The optical bench 10 is in thermal contact with the thermal interface 6of the TEC 4, e.g., it is bonded to the upper carrier plate 5, so as toensure a negligible thermal resistance between them. The surface area ofthe thermal interface 6 is such that the laser cavity is placed directlyabove the thermally stabilised upper carrier plate 5, i.e., both thegain medium 2 and the end mirror 3 are mounted on the surface area ofthe optical bench 10 placed above the upper carrier plate. In otherwords, the gain chip and the end mirror are placed at zero distance fromthe upper carrier plate 5 along the main longitudinal direction of theupper carrier plate, which is typically, but not necessarily,substantially parallel to the main optical axis of the optical beamwithin the laser cavity. Assuming that both the optical bench and thesubmount have a thermal conductivity not smaller than about 120 W/mK(such as in case of typical thermally conductive materials employed inlaser assemblies, for instance AlN or SiC), the end mirror is thermallystabilised at substantially T₁ (e.g., T₁±0.1° C.), while the temperatureof the gain medium can slightly differ from T₁, for example of about +1°C., due to heating of the semiconductor chip during operation.

In the case depicted in FIG. 1, mechanical deformations caused by thetemperature difference, ΔT=(T_(env)−T₁), induce an increase or decrease(depending on the sign of ΔT) of the optical path length, L_(opt).

The Applicant has noticed that the magnitude of the mechanicaldeformations increases with increasing the surface area of the uppercarrier plate, and thus of the thermal interface, of the TEC.Deformations along the longitudinal direction of the main optical axisof the optical beam within the laser cavity (i.e., the longitudinaldirection crossing the gain medium and the end mirror) affect theoptical path length of the laser cavity. Starting from that observation,the Applicant has considered a TEC of reduced surface dimensions. FIG. 2illustrates the case of a TEC 4′ having an upper carrier plate 5′ and alower carrier plate 8′. A Peltier cell 7′ is sandwiched between theupper and the lower carrier plate. The upper carrier plate 5′ has alength significantly smaller than that of the optical bench 10 mountingthe laser assembly so that both the gain chip and the end mirror are notplaced directly above the TEC upper carrier plate 5′. For instance, thesurface area of the TEC thermal interface 6′ is less than 50% smallerthan the surface area of the optical bench, although that value dependson several factors, such as the rigidity of the optical bench. The samereference numerals are given to elements of the laser modulecorresponding to those shown in FIG. 1 and their description is omitted.

A laser cavity in thermal contact with a TEC having a thermallystabilised surface much smaller than the area occupied by the lasercavity is largely unaffected by mechanical deformations. However, theuse of a TEC with a thermally stabilised surface area much smaller thanthe area occupied by the laser cavity can come at the cost of a reducedthermal stability. Thermal instability caused by the heat generatedduring operation of the gain medium and in general by a largersensitivity of the optical components to external thermal fluctuationsleads to an instability of the phase of the laser cavity and thus of thelasing frequency.

The Applicant has understood that the effect of mechanical deformations,i.e., the variation in the optical path length of the laser cavity, canbe at least partially compensated by adjusting the refractive index ofthe gain chip, said refractive index adjustments being obtained byacting upon the temperature difference between the temperature of thegain medium, T₂, and the environmental temperature, T_(env), so as toreduce (in absolute value) the temperature gradient ΔT′=(T_(env)−T₂).

In other words, Applicant has realised that the optical effect inducedby the temperature difference |ΔT|=|T_(env)−T₁| can be at leastpartially compensated by making the temperature of the gain medium, T₂,weakly dependent on T_(env)) i.e., T₂(T_(env)).

Heat transfer through a material, e.g., through the gain chip submountand/or the optical bench, can be expressed by using a thermal propertyof the material, which is its thermal resistance, R, characteristic notonly of the material but also of the geometry involved. The heattransfer can be compared to current flow in electrical circuits and thecombination of thermal conductivity, thickness of material andcross-sectional area can be considered as a resistance to this flow. Thetemperature difference is the potential for the heat flow and theFourier equation relating the heat flow rate, Q, to the temperaturegradient, ΔT can be written in a form similar to Ohm's law.

For a solid wall of thickness s and surface area A, the thermalresistance can be expressed by the following relationship:

$\begin{matrix}{{R = {\frac{\Delta\; T}{Q} = \frac{1}{\frac{\kappa}{s} \cdot A}}},} & (3)\end{matrix}$where κ is the thermal conductivity of the material, generally expressedin W/mK. Within the temperature range of interest, i.e., the operativetemperature range of an external cavity laser for telecommunications, κcan be assumed to be essentially independent of the temperature.

Heating of the gain medium generated during operation, which is roughlyproportional to the injection current provided to the chip, is to betaken into account. Such a heating, which can be referred hereafter toas self-heating, is independent of the environmental temperature, but itis dependent on the thermal resistance of the heat flow path to the TECthermal interface at about T₁.

To the purpose of reducing the temperature difference |ΔT′|, theApplicant studied a laser module having a configuration such thatdepicted in FIG. 1, where the submount 9 of the gain chip was made innickel (Ni), having a thermal conductivity, κ, of about 61 W/mK, whichis much smaller than the thermal conductivity of the submounts generallyused in laser modules, for instance SiC (κ=300-400 W/mK), BeO (κ=190-210W/mK) or AlN (κ=140-180 W/mK). The optical bench was of AlN. The thermalresistance of the Ni submount and the optical bench for the heat flowpath from the gain chip to the TEC thermal interface was of about 9-10K/W. In this way, the gain chip was not fully thermally stabilised,where full thermal stabilisation can be assumed to correspond to asituation in which the gain chip temperature varies with T_(env) withinabout 1° C. However, heat dissipation was observed to be poor and thetemperature on the gain chip was measured to be of 4-5° C. larger thanthe TEC temperature T₁, for environmental temperatures of 25° C. Asignificant temperature increase due to self-heating of the gain chipreduced the laser chip efficiency.

Applicant has found that “weak” dependence of the temperature of thegain medium on the environmental temperature with the purpose ofcompensating the effect of mechanical deformations induced byΔT=(T_(env)−T₁) can be obtained by thermally bridging the gain medium tothe environmental temperature, while maintaining a thermal couplingbetween the gain medium and a thermally stabilised surface area. Thermalbridge of the gain medium to the environmental temperature is achievedby the use of a thermal bridge element for conducting heat either fromor to the gain medium.

FIG. 3 illustrates an equivalent resistance circuit that can help inillustrating the principles underlying the present invention. The gainchip is at a temperature T₂ and its heat flow path to the thermallystabilised surface area of the TEC is given by the thermal resistanceR_(OB), e.g., the sum of the thermal resistance of the gain chipsubmount and of the optical bench. The thermal bridge between the gainmedium and the environmental temperature, T_(env) is defined by thermalresistance R_(TB).

The gain chip needs to be in thermal coupling to the TEC in order toprevent overheating during operation and in general a large sensitivityof the gain medium to thermal fluctuations. The heat flow path betweenthe gain medium and the TEC thermally stabilised surface is such thatits thermal resistance, R_(OB), is preferably smaller than about 10 K/W,and more preferably not larger than 8 K/W, so as a good heat dissipationcan be obtained.

Considering a laser assembly housed within a package enclosure, in thefollowing equations, the heat flow contribution of the heat transferwithin air from warms areas of the package (e.g., in proximity to thewalls of the package in case of T_(env)>T₁) to cooler areas (e.g., closeto the thermally stabilised surface area of the TEC, always forT_(env)>T₁) on the gain medium, i.e., heat convection, will bedisregarded. This assumption is not completely valid, but in practice,for the laser module configurations typically considered, it can beacceptable because the value of the thermal resistance originating fromheat convection, R_(c), is small when compared with the values of R_(OB)and R_(TB).

The power (in Watts) dissipated in the gain medium, P₀, is the sum oftwo contributions: the power dissipated along the heat flow path to thethermally stabilised surface, P_(OB), and the power dissipated along theheat flow path formed by the thermal bridge element, P_(env),

$\begin{matrix}\begin{matrix}{P_{0} = {P_{env} + P_{OB}}} \\{= {\frac{T_{env} - T_{2}}{R_{TB}} + \frac{T_{2} - T_{1}}{R_{OB}}}} \\{{= {\frac{\Delta\; T}{R_{TB}} - \frac{\Delta\; T_{OB}}{R_{TB}} + \frac{\Delta\; T_{OB}}{R_{OB}}}},}\end{matrix} & (4)\end{matrix}$where ΔT=(T_(env)−T₁) and ΔT_(OB)=(T₂−T₁). Within the typical operativetemperature range of laser modules for telecommunications, e.g., −5° C.to 70° C., the temperature difference ΔT can be a positive or a negativevalue (or zero, when T_(env)=T₁), whereas ΔT_(OB) is generally notsmaller than zero because of self-heating of the laser chip duringoperation.

Equation (4) can be re-written as

$\begin{matrix}\begin{matrix}{{\Delta\; T_{OB}} = {{\frac{R_{OB} \cdot R_{TB}}{R_{TB} - R_{OB}} \cdot P_{0}} - {{\frac{R_{OB}}{R_{TB} - R_{OB}} \cdot \Delta}\; T}}} \\{= {\alpha\left( {{R_{TB} \cdot P_{0}} - {\Delta\; T}} \right)}}\end{matrix} & (5)\end{matrix}$where

$\begin{matrix}{\alpha = {\frac{R_{OB}}{R_{TB} - R_{OB}}.}} & (6)\end{matrix}$

Applicant has found that in order to realise a weak dependence of thetemperature, T₂, of the gain medium while ensuring a thermal coupling tothe TEC such that excessive self-heating (e.g., >5° C. across theoperative temperature range) is avoided, thermal resistance of the heatflow path to the TEC thermal interface, R_(OB), should be smaller thanthe thermal resistance of the thermal bridge element, R_(TB).Preferably, the value of R_(TB) is at least about 1.5 times the value ofR_(OB), i.e., R_(TB)=c·R_(OB), with c being at least about 1.5. Morepreferably, c is at least about 2 and even more preferably c iscomprised between 5 and 12.

In general, if T_(env)>>T₁ (e.g., ΔT>+20° C.), the gain medium is at atemperature, T₂, closer to the environmental temperature than thetemperature T₁. Conversely, if T_(env)<T₁, the gain chip is at atemperature, T₂≧T₁ so that the temperature gradient between the gainchip and the stabilised temperature, ΔT_(OB), is generally either zeroor of opposite sign to the temperature gradient between T₁ and T_(env).Both for T_(env)>T₁ and for T_(env)<T₁, a variation of the optical pathlength is induced in the direction opposite to the variation of theoptical path length caused by mechanical deformations.

When T_(env)>T₁, mechanical deformations lead to a reduction of thelaser cavity optical length, said reduction being approximatelyproportional to the temperature difference ΔT=(T_(env)−T₁)>0. In thiscase, the temperature difference between the gain chip and theenvironmental temperature, ΔT′=(T_(env)−T₂), is often not smaller thanzero, but it is smaller than ΔT. The situation of ΔT′=0 corresponds tothe case where T₂ is accidentally equal to T_(env), such a situation canoccur generally when T_(env) is larger than T₁ of not more than about10° C. Increasing the temperature of the gain medium with respect to thestabilised temperature produces a variation of the refractive index ofthe gain chip with consequent increase of the optical path length.

When T_(env)<T₁, mechanical deformations of the optical bench induce anincrease of the optical path length approximately proportional to thetemperature gradient ΔT=(T_(env)−T₁)<0. There is a temperature gradient,ΔT'=(T_(env)−T₂)≦0. Therefore, a reduction of the optical path length ofthe laser cavity, which compensates the increase of the optical pathlength caused by mechanical deformations, is achieved.

Preferably, the thermal resistance of the heat flow path from the gainmedium to the thermally stabilised surface of the TEC is not smallerthan about 5 K/W. For conventional choices of thermally conductivematerials and their dimensions in the laser module, the smaller thevalue of thermal resistance R_(OB), the larger the cross-sectional area(“A” in Eq. (3)) of the thermal bridge element required to produce thedesired temperature difference, ΔT′, on the gain medium.

One way to achieve the desired value of the thermal resistance, R_(OB),is by placing the gain chip not above the TEC thermally stabilisedsurface, but at a certain distance from it. According to a preferredembodiment, the gain medium is placed on a thermally conductive platformhaving a length larger than the length of the TEC thermally stabilisedsurface along the longitudinal direction of the optical axis of thelaser beam within the cavity. Preferably, the gain medium is placed at acertain distance, d₁, along said longitudinal direction, from thethermally stabilised surface of the TEC. Assuming an upper carrier plateof the TEC with a substantially rectangular surface and a laser cavitybeing arranged so that the main longitudinal optical axis issubstantially parallel to a first side edge of the rectangular surface,the distance d₁ is essentially parallel to said first side edge of theTEC upper plate.

It is to be noted that compensation of the variation of the optical pathlength of the laser cavity according to the present invention can bealso partial, i.e., in the direction of a significant decrease of saidvariation without reducing it to zero. Also in case partial compensationis achieved by the present solution, it will be possible to achieve fullcompensation by making only small adjustments of the injection currentof the gain medium, thereby avoiding creation of relatively largefluctuations in the laser output optical power. Preferably, injectioncurrent adjustments are within about ±10 mA and more preferably within±15 mA.

Preferably, the thermal bridge element is structured so as tomechanically disconnect the gain chip from the housing of the package inorder to avoid damaging or breaking of the device when the package issubjected to shock or inertial forces. Preferably, the thermal bridgeelement is a resilient element.

The present invention has the advantage that compensation of thevariations of the optical path length of the laser cavity is carried outpassively, without the need of introducing an active thermal controland/or an extra electronic control circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating a laser external cavitythermally stabilised by means of a thermoelectric cooler.

FIG. 2 is a schematic diagram illustrating the laser external cavity ofFIG. 1, in which thermal stabilisation is lessened due to athermoelectric cooler of reduced dimensions with respect to the lasercavity.

FIG. 3 illustrates an equivalent resistance circuit representing theheat flow path to and from the gain medium of an external laser cavityaccording to the invention.

FIG. 4 is a block diagram of the external-cavity laser assemblyaccording to an embodiment of the present invention.

FIG. 5 shows exemplary measurements of the laser output power as afunction of the injection current for different environmentaltemperatures.

FIG. 6 shows computer simulations of the temperature as a function ofthe length of an optical bench mounting an external cavity along themain longitudinal direction of the optical bench, which is substantiallyparallel to the main optical axis of the optical beam within the cavity.

FIG. 7 is a lateral view of a laser module according to a firstpreferred embodiment of the present invention.

FIG. 8 is a top plan view of the laser module illustrated in FIG. 7.

FIG. 9 is a cross-sectional view of a laser module along line AA of FIG.8, in which a mechanical schematic cross-sectional view of a thermalbridge element is shown.

FIG. 10 is a portion of the cross-sectional view of FIG. 9 illustratingthe situation where an external force is exerted on the packagefeed-through.

FIG. 11 is a mechanical perspective schematic view of a thermal bridgeelement according to an embodiment of the present invention.

FIG. 12 is a mechanical perspective schematic view of a thermal bridgeelement according to another embodiment of the present invention.

FIG. 13 is a mechanical perspective schematic view of a thermal bridgeelement according to a further embodiment of the present invention

FIG. 14 is a partial perspective view of a laser module according to afurther embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

According to the preferred embodiments of the present invention, theexternal-cavity laser is a tuneable laser including a gain mediumemitting a light beam, a grid generator and a channel selector, bothgrid generator and channel selector being located along the optical pathof the beam exiting the gain medium. The grid generator selects periodiclongitudinal modes of the cavity at intervals corresponding to thechannel spacing and rejects neighbouring modes. The channel selectorselects a channel within the wavelength grid and rejects other channels.

Preferably, the channel selector is a tuneable mirror, which forms anend mirror of the external cavity and defines it in length together withthe reflecting front facet of the gain medium, e.g., a semiconductorlaser diode. In other words, the tuneable mirror functions both aschannel selector and as end mirror for the laser cavity.

The grid generator is preferably a Fabry-Perot (FP) etalon, which isstructured and configured to define a plurality of equally spacedtransmission peaks. In applications for WDM or DWDM telecommunicationsystems, transmission peak spacing, i.e., the free spectral range (FSR)of the grid element, corresponds to the ITU channel grid, e.g., 50 or 25GHz.

When present in the laser cavity together with the grid generator, thetuneable mirror serves as the coarse tuning element that discriminatesbetween the peaks of the grid generator. For single-mode laser emission,a longitudinal cavity mode should be positioned over the maximum of oneof the grid transmission peaks (the one selected by the tuneablemirror). In this way, only the specified frequency will pass through thegrid and the other competing neighbouring cavity modes will besuppressed. Wavelength selectivity of the tuneable mirror is preferablyachieved by an electrical signal. The tuneable mirror of the presentinvention preferably comprises an electro-optically tuneable material,more preferably a liquid crystal (LC) material. In case of a LC-basedtuneable mirror, the applied voltage is an ac voltage.

FIG. 4 schematically illustrates the layout of an external-cavity lasercomprising a tuneable mirror. Gain medium 11 comprises a front facet 13and a back facet 12. Back facet 12 is partially reflecting and serves asone of the end mirrors of the external cavity. Front facet 13 has a lowreflectivity. It is typically coated with an anti-reflection coating(not shown). A collimating lens 14 converges the optical beam emitted bythe gain medium onto a FP etalon 15, which has the modes locked to theITU channel grid. After the FP etalon 15, the beam impinges on atuneable mirror 16, which forms the other end mirror of the externalcavity and together with the gain medium back facet defines the cavityphysical length, L_(o). The tuneable mirror is tuned to the desiredchannel frequency by selecting one of the etalon transmission peaks. Thetuneable mirror 16 is tuned electronically by varying the appliedvoltage supplied by a voltage generator 17.

For the laser effect to occur in the laser cavity, two conditions shouldbe met: the condition for resonance and the condition for gain, whichcan be expressed, respectively, by the following equations2jΦ _(LD)+2jΦ _(FS)+2jΦ _(FP) +jΦ _(r2)=2jNπ  (7)G _(LD) ²(λ)G _(FP) ²(λ)·r ₁ ·r ₂(λ)=1  (8)where N is an integer number, G_(LD) is the spectral gain of the laserdiode, φ_(LD) is the phase delay introduced by the laser diode, φ_(FP)is the phase delay introduced by the etalon, φ_(FS) is the phase delayintroduced by the free space, G_(FP) is the transmission spectrum of theetalon, r₁ is the reflectivity of the front facet of the laser diode,r₂(λ) is the reflectivity of the tuneable mirror, and φ_(r2) is thephase delay introduced by the tuneable mirror.

Equations (7) and (8) can be combined to obtain the laser cavity modesG _(LD) ²(λ)e ^(2jΦ) ^(LD) ·e ^(2jΦ) ^(FS) ·G _(FP) ²(λ)·e ^(2jΦ) ^(FS)·r ₁ ·r ₂(λ)·e ^(jΦ) ^(r2) =e ^(2jNπ)  (9).

FIG. 5 shows a few exemplary measurements of the laser output power as afunction of the injection current of the laser diode for three differentenvironmental temperatures, T_(env). The laser configuration isgenerally similar to that in FIG. 2, but with the end mirror positionedon the optical bench in correspondence to the TEC upper carrier plate.The gain chip was positioned in the laser cavity such that the thermalresistance, R_(OB), to the thermally stabilised TEC was of about 6.5-7K/W. It is clear from FIG. 5 that an increase of the environmentaltemperature leads to a shift of the local maxima of the output powertowards higher injection currents. This effect is deemed to be mostlydue to a decrease of the physical length of the laser cavity, L₀, andthus of φ_(FS). Assuming that also the FP etalon and the tuneable mirrorare thermally stabilised, contributions from the etalon and of thetuneable mirror to the temperature dependence of the phase delay of thelaser cavity can be disregarded.

If the gain chip is fully thermally stabilised (e.g., within ≈1° C.),its phase delay,  _(LD), depends essentially on the injection current,I_(LD). The local maxima in the output power correspond to the conditionof optical power alignment of the cavity modes with the (selected)etalon peak.

In case of occurrence of mechanical deformations of the TEC, which areinduced by variations of the environmental temperature, T_(env) thefree-space phase delay, φ_(FS), undergoes variations and then Eq. (7)can be written by highlighting the temperature dependence of φ_(FS) andthe dependence of the phase delay of the laser diode on the injectioncurrent as2jΦ _(LD)(I _(LD))+2jΦ _(FS)(T _(env))+2jΦ _(FP) +jΦ _(r2)=2jNπ  (10)

Compensation of the variation of the free-space phase delay to achieveprecise channel alignment of the cavity mode could be thus attained byadjusting the injection current of the laser diode. In other words, byadjusting the injection current, the position of cavity mode can becentred under the selected etalon peak (i.e., lasing channel) therebybringing the output power to a local maximum (FIG. 5). Monitoring theoutput power could therefore be employed for adjustments of channelalignments so that the following relationship is satisfied:Φ_(LD)(I _(LD))+Φ_(FS)(T _(env))=constant  (11).

However, Applicant has noted that within a typical operative temperaturerange of the laser assembly, i.e., −5° C., +70° C., variations of theinjection current necessary to satisfy Eq. (11) can be up to about 40 oreven 60 mA. Such current variations lead to variations in the outputoptical power as high as about 1 dB, clearly unacceptable when a highstability of the laser output is required.

The Applicant has considered acting upon the phase delay of the gainchip by thermally varying the refractive index of the gain chip so as tocompensate the variation of the free-space phase delay caused bymechanical deformations of the TEC. It has been found that thermallybridging the gain medium to the environmental temperature so that theheat flow path to T_(env) has a thermal resistance, R_(TB), larger thanthat of the heat flow path from the gain medium to T₁ (the thermallystabilised surface) can compensate, at least partially, the variation ofthe optical path length due to mechanical deformations.

FIG. 6 shows computer simulations of the temperature curves vs. lengthof the optical bench mounting a laser assembly. Optical bench 56 isplaced on a TEC 55 with surface area of the thermally stabilised surfaceof 8×8 mm² (i.e., the TEC upper carrier plate is a square-shaped plate)said surface being stabilised at a temperature T₁=25° C. Simulationswere carried out at two different environmental temperatures, T₁=25° C.(i.e., equal to the temperature at which the TEC is set) and T_(env)=70°C. The optical bench is a rectangular-shaped plate of length, 1, of 17.5mm (along the Y-axis) and width of 7.2 mm. The optical bench 56 isplaced on the TEC upper plate at two different positions along thelongitudinal main direction of the optical bench (i.e., Y axis), whichcorrespond to two different distances, d, along the Y direction, betweenthe edge of the optical bench and the edge of the TEC upper carrierplate. The optical axis of the laser cavity is substantially parallel tothe Y direction. The two distances d, which are 5 and 8 mm, in turncorrespond to two different distances, d₁, of about 1 mm and 4 mm,respectively, of the gain chip from the upper plate of the TEC. Theoptical bench 56 is assumed to have a thermal conductivity of about 180W/mK, while the submount 57 is assumed to have a thermal conductivity of160-190 W/mK. In simulations, the laser cavity included a FP etalon 59and a tuneable mirror 60. Both the FP etalon and the tuneable mirror areplaced on a surface area of the optical bench at a zero longitudinaldistance (i.e., along the Y-axis) from the thermally stabilised surfaceof the TEC 55.

A slight increase (within about 1° C.) of the temperature incorrespondence to the gain chip can be seen also for T_(env)=T₁ (solidtriangles and solid circles in FIG. 6), due to self-heating of the gainchip during operation and to the fact that the gain chip is positionedat d₁ from the TEC thermally stabilised surface. It is clear from FIG. 6that to larger distances d₁ there correspond larger temperaturegradients on the gain chip. For T_(env)=70° C. (ΔT=45° C.) and d₁=4 mm,ΔT_(TB)≈1.7° C. If the distance d₁ is smaller than about 2 mm (d₁=0—notshown in the figure—corresponds to the case the gain chip is placeddirectly above the TEC upper carrier plate), the gain chip undergoesrelatively small temperature variations with T_(env) for ΔT>>0 (i.e., asmall reduction of ΔT′=(T_(env)−T₂) is obtained), which can be notsufficient to significantly compensate the variations of optical pathlength due to mechanical deformations of the TEC.

Applicant has observed that also in case of a gain chip placed at arelatively large distance from the closest side edge of the TEC upperplate, e.g., d₁=4 mm, the temperature gradient can be not sufficient tobring a beneficial contribution to the reduction of the optical pathlength variation. Since larger longitudinal distances from the edge ofthe TEC are often not feasible in designs of commercial external-cavitylasers, such as in case of transmitters for WDM/DWDM systems (referenceis made also to Eq. (1)) and/or because of package constraints,Applicant has found that the temperature gradient on the gain chip, ΔT′,can be reduced by thermally bridging the gain chip with theenvironmental temperature, i.e., by the use of a thermal bridge elementthat thermally connects the gain chip to a component placed at about theenvironmental temperature, while maintaining a thermal coupling betweenthe gain chip and the TEC.

Laser assemblies are typically housed in a package that protects thelaser components and other electronic or thermoelectric componentsassociated to the laser assembly from the external environment. Externalcavity lasers for telecommunications are generally housed inhermetically sealed packages so as to allow the laser assembly to besealed within an inert atmosphere to prevent contamination/degradationof the optical surfaces of the various components of the laser.

It has been found that a thermal bridge element can thermally couplesthe gain chip to a portion or a component of the package placed at aboutthe environmental temperature. Thermal exchange between the gain chipand the package portion at about the environmental temperature dependson the thermal resistance of the bridge element, which depends on thethermal conductivity of the material forming the element, itscross-sectional area and its length, equivalent to thickness “s” in Eq.(3).

It would be in principle possible to obtain a temperature difference onthe gain chip by employing a thermal bridge element in a laser cavitywherein the gain chip is placed above the TEC (as for instance depictedin FIG. 1), when optical benches and gain chip submounts are made ofmaterials with κ larger than about 120 W/mK. For instance, a lasermodule with a configuration of FIG. 1, i.e., the gain chip is placedabove the TEC on an AlN submount on an AlN optical bench, can have athermal resistance R_(OB) of about 2-3 K/W, depending on the dimensionsand thermal conductivity of the optical bench and submount. However,this would require the use of a bridge element having a rather smallthermal resistance, R_(TB), e.g., having a rather large cross-sectionalarea. In most commercial packages housing tuneable lasers fortelecommunications, the large dimensions of the thermal bridge elementnecessary to produce a significant temperature gradient (e.g., ΔT′≈±2-3°C.) would render employment of a thermal bridge element unfeasible.Furthermore, a large cross-section is less preferred because to a largercross-section of the thermal bridge element there corresponds a largerrigidity of the element.

It is therefore preferred that the thermal resistance of the heat flowpath between the gain chip and T₁, R_(OB), is not smaller than 5 K/W.

A tuneable laser module according to a preferred embodiment of thepresent invention is schematically depicted as a side view in FIG. 7.The laser module 20 comprises an external cavity laser assembly housedin a package 26, e.g., a 14-pin butterfly package, which defines anenclosure 51 including four lateral walls (only two walls are shown inFIG. 7). The package 26 comprises a base 54 and a lid 50. The lid sealshermetically the enclosure. The package includes a boot 21 for theinsertion of an optical fibre, i.e., fibre pigtail 23. A glass window 25closes up hermetically the laser assembly from the boot for fibreinsertion. The laser assembly includes a gain medium 35, a collimatinglens 37, a grid generator 39, a deflector 42 and a tuneable mirror 45.The gain medium 35 is based on a semiconductor laser chip, for examplean InGaAs/InP multiple quantum well Fabry-Perot (FP) gain chipespecially designed for external-cavity laser applications. The diodecomprises a back facet 36 and a front facet 46. The diode's front facet46 is an intracavity facet and has an anti-reflection coating.Preferably, the gain chip waveguide is bent so that it has an angledincidence on the front facet in order to further reduce backreflections. The back facet 36 is partially reflective and serves as oneof the end mirrors of the external cavity. The reflectivity of the backfacet can range for instance between 10% and 30% in order to allow arelatively high laser output power.

Within the laser cavity, the emerging beam from the laser chip frontfacet 46 is collimated by collimating lens 37 that collimates the beamto define an optical path 47. The collimated beam impinges onto gridgenerator 39.

The laser assembly is placed on a thermally conductive platform 30,i.e., the optical bench, which functions also as mechanical referencebase for the optical elements. The use of a common optical bench ispreferred because it minimises the design complexity and simplifies thealignment between the components of the tuneable laser. The platform 30is made of any thermally conductive material, such as aluminium nitride(AlN), silicon carbide (SiC) and copper-tungsten (CuW).

The thermally conductive platform 30 is thermally contact with a TEC 44including an upper carrier plate 44 a, a Peltier cell 44 b and a lowercarrier plate 44 c. TEC 44 provides thermal control for the platform bystabilising the temperature of the upper carrier plate 44 a, which is inthermal contact with the optical bench 30. For instance, the platform 30can be glued or soldered to the upper carrier plate 44 a so as tominimise the thermal resistance between the surface where the opticalcomponents are mounted and the thermally stabilised surface. The lowercarrier plate 44 c is secured on the base 54 of the package. Temperaturemonitoring of the thermally conductive platform is provided by a thermalsensor device 33, such as a thermistor or a thermocouple, which isplaced on the platform and is operatively coupled to the TEC so as toprovide control signals to cool or heat the surface of the TEC inthermal contact with the platform 30, and thus to heat or cool platform30 in order to maintain an approximately constant temperature, T₁. Inthe embodiment of FIG. 7, the thermal sensor device is placed inproximity of the FP etalon 39, for control of its thermal stability. Thelower carrier plate 44 c of the TEC 44 is placed at about theenvironmental temperature, T_(env), on the internal surface (i.e.,internal with respect to the enclosure) of base 54.

The gain chip 35 is preferably placed, e.g., by bonding, on a thermallyconductive submount 34 so as to position the emitted beam at aconvenient height with respect to the other optical elements and tofurther improve heat dissipation. The thermally conductive sub-mount 34,made for instance of SiC, is placed on the thermally conductive platform30.

The length of the platform 30 substantially along the main longitudinaldirection Y is substantially parallel to the main optical axis of theoptical beam within the laser cavity and it is larger than the length ofupper carrier plate 44 a (always along the Y direction). The gain chip35 is placed at a certain longitudinal distance, along Y, from the edgeof the upper plate of the TEC. The position of the gain chip is selectedso that a temperature difference, ΔT′, not larger than 5° C. is createdbetween the gain chip temperature and T_(env) for an environmentaltemperature largely different (e.g., more than 10-15° C. and typicallynot larger than 45-50° C.) from the stabilised temperature, T₁.Preferably, ΔT′ is comprised between 2° C. and 5° C., depending also onthe magnitude of the mechanical deformations to be compensated.

The grid generator 39 is preferably a FP etalon. The laser can bedesigned in such a way that the operating wavelengths are aligned withthe ITU channel grid. In this case, the laser wavelength is centred tothe ITU grid via the FP etalon 39, which is structured and configured todefine a plurality of equally spaced transmission peaks. In applicationsfor WDM or DWDM telecommunication systems, transmission peak spacing,i.e., the FSR of the grid element, corresponds to the ITU channel grid,e.g., 50 or 25 GHz. In order to stabilise its temperature, the FP etalon39 is preferably housed in a thermally conductive housing 38 to promotethermal contact with the platform 30.

After the FP etalon 39, the laser beam strikes a deflector 42 thatdeflects the beam 47 onto a tuneable mirror 45 along optical path 49.The tuneable mirror 45 reflects the light signal back to the deflector42, which in turn deflects the light signal 48 back to the gain medium35. The deflector 42 is in this embodiment a planar mirror, for instancea gold-coated silicon slab.

Although not shown in FIG. 7, the deflector can be secured in the cavityfor instance by means of a support structure that is fixed to theplatform 30. The deflector can be glued to the support structure or, ifit is at least partly metallised, soldered. Preferably, the deflector isaligned to the laser beam by means of active optical alignmenttechniques.

The tuneable mirror 42 is an electro-optic element, in which tunabilityis achieved by using a material with voltage-dependent refractive index,preferably a liquid crystal (LC) material. For example, a tuneablemirror is that described in WO patent application No. 2005/064365. Thetuneable mirror serves as the coarse tuning element that discriminatesbetween the peaks of the FP etalon. The FWHM bandwidth of the tuneableelement is not smaller than the FWHM bandwidth of the grid etalon. Forlongitudinal single-mode operation, the transmission peak of the FPetalon corresponding to a particular channel frequency should select,i.e., transmit, a single cavity mode. Therefore, the FP etalon shouldhave a finesse, which is defined as the FSR divided by the FWHM, whichsuppresses the neighbouring modes of the cavity between each channel.For single-mode laser emission, a longitudinal cavity mode should bepositioned over the maximum of one of the etalon transmission peaks (theone selected by the tuneable element). In this way, only the specifiedfrequency will pass through the etalon and the other competingneighbouring cavity modes will be suppressed.

According to the present embodiment, the external laser cavity is afolded resonant cavity having an optical path length, which is the sumof the optical path 47 between the back facet 35 of the gain medium andthe deflector 42 and the optical path 49 between the deflector and thetuneable mirror 45.

The laser beam is coupled out of the external cavity by the partiallyreflective back facet 36 of the laser diode 35. Preferably, acollimating lens 32 can be placed along the optical path of the laseroutput beam. In the present embodiment, a beam splitter 29, e.g. a98%/2% tap, which is placed after lens 32, picks off a portion of theoutput light as a test beam, which is directed to a photodetector 31 forpower control. A fibre focus lens 22 directs the light, which has passedthrough an optical isolator 24, into fibre pigtail 23. Optical isolator24 is employed to prevent back-reflected light being passed back intothe external laser cavity and is generally an optional element.

In the preferred embodiments, the laser assembly is designed to producesubstantially single longitudinal and, preferably, single-transversalmode radiation. Longitudinal modes refer to the simultaneous lasing atseveral discrete frequencies within the laser cavity. Transversal modescorrespond to the spatial variation in the beam intensity cross sectionin the transverse direction of the lasing radiation. Generally, anappropriate choice of the gain medium, e.g., a commercially availablesemiconductor laser diode including a waveguide, guarantees singlespatial, or single transversal, mode operation. The laser is operativeto emit a single longitudinal mode output, which depends on the spectralresponse of the optical elements within the cavity and on the phase ofthe cavity.

Although not shown in FIG. 7, lenses 32 and 37 are mounted to theplatform by individual mounts.

The FP etalon 39 and the tuneable mirror 45 are mounted on the surfacearea of the optical bench 30 placed above the upper carrier plate 44 aof the TEC 44 in order to minimise the thermal resistance of the heatflow path. The thermal resistance of the heat flow path between thetuneable mirror and the TEC surface is preferably smaller than thethermal resistance, R_(OB), between the gain chip and the TEC surface.The thermal resistance between the tuneable mirror and the TEC thermallystabilised surface is preferably not larger than 2 K/W, more preferablynot larger than 1 K/W.

The tuneable mirror 45 lays substantially horizontally with respect tothe principal surface plane of the thermally conductive platform 30. Ina preferred embodiment, the tuneable mirror is placed onto a thermallyconductive substrate or in a holder (indicated with 41 in FIG. 7) thatcan house the tuneable mirror. In case the platform 30 is made of ametallic material, the substrate or holder 41 should be made of anelectrically insulating material (with high thermal conductivity) inorder to avoid an electrical contact between the tuneable mirror and theplatform, As the tuneable mirror is normally biased during laseroperation. In a preferred embodiment, the holder 41 is made of AlN orSiC.

By laying the tuneable mirror horizontally on the platform, the thermalcontact with the platform is maximised while there is no need ofactively aligning the mirror with respect to the laser beam during laserassembly. Preferably, during laser assembly, the tuneable mirror isbonded onto the thermally conductive platform by means of a thermallyconductive epoxy, for instance Ag-filled epoxy, or of silicone resin.Alternatively, the tuneable mirror is housed in a holder or placed on asubstrate that is bonded to the thermally conductive platform. Themirror can be glued to the substrate or holder. What needs to be alignedto the laser beam, preferably by optical active alignment techniques, isthe deflector 42.

For a laser configuration having the tuneable mirror “horizontal” withrespect to the optical bench, maximum temperature variations of thetuneable mirror remain lower than 0.1° C. across the temperatureoperating range from −10 to 70° C., even when the dissipated power ofthe tuneable mirror, which is due to the applied voltage, is as high as50 mW.

It is however to be understood that the present invention envisage asalternative preferred embodiment also an external cavity laser assembly,wherein the tuneable mirror is positioned substantially perpendicularlyto the optical beam (as schematically shown in FIG. 4).

FIG. 8 is a top plan view of the laser module shown in FIG. 7. The lid(not shown) of the case is omitted in FIG. 8 for clarity. The samereference numerals are given to elements of the tuneable laser modulecorresponding to those shown in FIG. 7 and their explanation will beomitted. In the embodiment shown in FIGS. 7 and 8, the package is abutterfly package including a plurality of electrical leads (i.e., pins)for electrical connections of the components in the interior of thehousing 51, such as the gain medium, the TEC, the tuneable mirror, andpossibly to other components such as the thermistor 33. The electricalleads 28 are soldered on the electrically conductive pads 52 of afeed-through ceramic structure 27 that provides an electricalinterconnect into or out of the enclosure 51, while maintaining thepackage hermeticity.

A thermal bridge element 43 is placed on the submount 34 in proximity ofthe gain chip 35. The thermal bridge element 43 thermally couples thegain chip to a region of the package that is in thermal contact with theenvironmental temperature. In particular, the thermal bridge element 43has two peripheral ends 43 a and 43 b, a first end, 43 b, is placed inproximity of the gain chip 35, whereas a second end, 43 a, is placed ona portion of the package.

Ideally, the thermal bridge element should be placed in physical contactwith the gain chip so as to maximise the thermal effect. The chipsubmount, being made of a thermally conductive material, tends toquickly dissipate the heat conducted along the thermal bridge element.However, some laser chips, such as a standard InGaAs semiconductor laserchips, have relatively small dimensions (e.g., for example a surfacearea of 0.5×1 mm²) and are rather easily breakable, which makes oftenundesirable to bond/solder the thermal bridge element directly on thegain chip. In such cases, the thermal bridge is placed on the gain chipsubmount in close proximity, e.g., within 2 mm from an edge of the gainchip.

The region of the package that is in thermal contact with theenvironmental temperature on which an end of the thermal bridge elementis placed is preferably a surface portion of the package feed-through27, in particular on its upper surface, as illustrated in FIG. 8. Forinstance, an end of the thermal bridge is soldered by known methods on ametallic bonding pad 52 of the feed-through 27. Although it is preferredthat the thermal contact of the gain chip with the package is made withthe package feed-through, because of its relatively small physicaldistance with the gain chip, it is to be understood that differentconfigurations of the thermal bridge element and/or of the position ofthe gain chip within the package can include a thermal connectionbetween the gain chip and the package, e.g., the base 54 or a lateralwall of the enclosure 51.

The thermal bridge element is made of a thermally conductive material,for example of Cu having a thermal conductivity of about 400 W/mK. Thesmaller the value of the thermal conductivity, the larger needs to bethe cross-sectional area of the thermal bridge to maintain the desiredthermal resistance, according to Eq. (3). However, a largercross-sectional area is often not preferred because it increases therigidity of the thermal bridge and its encumbrance. A preferred materialis copper. In the embodiment described with reference to FIGS. 7 and 8,the thermal bridge element is made of soft copper, which has arelatively low Young's modulus of about 100 GP (i.e., low mechanicalrigidity), which provides for a good mechanical workability. A coatingof gold or silver can be formed on the copper element in order tofacilitate the soldering of the bridge element to the packagecomponents, such as the solder pads 52 of the ceramic feed-through 27.

The thermal bridge element 43 can be soldered to the feed-through 27 andto the submount 34 by means of known soldering techniques, for exampleby using eutectic soldering alloys, such as AuSb.

FIG. 9 is a cross-sectional view along line AA of FIG. 8. A transversalcross-sectional view of the thermal bridge element 43 shown in thefigure.

The thermal bridge element 43 has a resilient structure so as tomechanically de-couple the gain chip, and in general the optical bench,from external stresses transmitted through the external walls of thepackage housing or the feed-through/pins of the package. In other words,the thermal bridge element has preferably a spring-like behaviour alongthe longitudinal direction indicated by arrow 53.

FIG. 10 schematically illustrates the situation when a force along arrow60 is exerted on the package feed-through structure 27, and thus on alateral wall of enclosure 51 fitting the feed-through. The thermalbridge element 43 in response to the force undergoes a lateralmechanical compression, thereby avoiding damaging or breaking the gainchip.

FIG. 11 is a perspective view of the thermal bridge element 43 shown inthe embodiment of FIGS. 7-9. The thermal bridge element 43 includes twoflat arms 66 and 67 and a central resilient portion 68 having a reversedV-shaped longitudinal cross-section. Thermal bridge element 43 cantherefore be thought of as being made of four straight portions, two ofthem (i.e., arms 66 and 67) being substantially parallel to one another.From Eq. (3), the thermal resistance, R_(TB), of the bridge elementdepends on the thermal conductivity of the material, on itscross-sectional area, A=t·a, where t is the thickness of the metalsheet, and on the length of the material traversed by the heat flow,i.e., the longitudinal development of the bridge element. Thelongitudinal development of the bridge element is the sum of the lengthsof the two arms, L₁ and L₂, and of reversed V-shaped portion (≈2L₃). Fora defined material, thermal resistance is inversely proportional to thethickness, t, i.e., to the rigidity of the element (generally expressedby the Young's modulus). On the other hand, the thermal resistance ofthe bridge element increases linearly with its length (longitudinaldevelopment). Therefore, in practice, when resiliency of the element isdesired, an optimal preferred cross-sectional area of the thermal bridgeelement is a compromise between the two requirements. It is further tobe noted that with increasing the longitudinal development of the bridgeelement may lead to an increase of heat dissipation for convection, dueto heat exchange by convection between the thermal bridge element andthe surrounding air.

The bridge element 43 is attached to the components of the packagemodule on the flat arms 66 and 67 (as shown for instance in FIG. 9). Theelement 43 is made of a relatively thick metallic sheet (e.g., ofthickness, t, of 0.25 mm) manufactured for instance by photo etching(chemical machining on plate).

To improve mechanical flexibility of the thermal bridge element, thecentral portion of the element can be made for instance of a shape ofreversed W, as shown in FIG. 12. The thermal bridge element 70 comprisestwo straight portions 71 and 72 and a resilient central portion 73having a reversed W-shaped cross-section, which includes two flexiblesub-portions, 74 and 75, each having a reverse V-shape. By increasingthe number of the flexible V-shape sub-portions, resiliency of thebridge element increases. However, package constraints can pose an upperlimit on the longitudinal development of the bridge element and thus ofits longitudinal dimension (i.e., along the X direction), which forinstance cannot be larger than 3-4 mm.

An efficient resilient structure of the thermal bridge element withreduced encumbrance and relatively large thermal resistance has beenstudied with the help of computer simulations by the Applicant.

FIG. 13 shows a perspective view of a preferred embodiment of thethermal bridge element. Thermal bridge element 81 includes four straightportions: a first and a second arm, 76 and 77, respectively, said firstand second arms laying on two substantially parallel planes having anoffset h along their perpendicular direction, and first and secondcentral portion, 79 and 80, abutted at one edge at a given angle so asto form a reversed V-shaped structure. The offset between the first andsecond arm is designed in order to take into account the heightdifference between the position at which the chip is placed (i.e.,basically the submount height) and the position of the package. The fourstraight portions of the thermal bridge element 81 are made of ametallic sheet, wherein the metal is preferably soft copper. In order toimprove resiliency of the bridge element, grooves 78 are formed at thehinges between the different straight portions.

Selection of suitable dimensions, shape and material of the thermalbridge element depends on several factors, among which the design of thelaser cavity and the thermal resistance between the gain chip and thethermally stabilised TEC surface area (approximately corresponding tothe thermally stabilised surface area of the optical bench), R_(OB). Byknowing these parameters, variation of the optical path length of thelaser cavity due to mechanical deformations of the TEC can be estimated,e.g., by computer simulations, for the operative range of the externalcavity laser, i.e., ΔΦ_(FS)(T_(env)) from Eq. (11) can be estimated forthe operative temperature range ΔT. The necessary compensation to thevariation ΔΦ_(FS)(T_(env)) is thus found, either partial or fullcompensation. A thermal bridge element can be designed to produce thenecessary heating/cooling of the gain chip for compensation of theoptical path variation.

For example, compensation of the variation of the cavity optical lengthis found to be achieved by a temperature difference on the gain chip of0.055° C./° C., i.e., for a variation of +45° C. of the environmentaltemperature (e.g., from 25 to 70° C.) a heating of 2.5° C. of the gainchip is to be induced. A thermal bridge element of Cu inducing such atemperature gradient on the gain chip placed at d₁=3 mm from the TECexposed surface can have the structure described in FIG. 13 and thefollowing dimensions: the transversal length of the bridge element is of3.65 mm; first and second arm are 0.87 mm long and 0.50 wide; the offseth is of 0.45 mm; the two central portions form an angle, β, of 55° withrespect to the parallel planes on which the two arms lie, and themetallic sheet making the four portions is 0.25 mm thick. The thermalresistance, R_(TB), is of about 80 K/W.

According to an alternative embodiment, the thermal bridge element is ametallic wire made of a material with high thermal conductivity, such ascopper. For example, a Cu wire of 0.25 mm of diameter and 1.5 mm long,having a thermal resistance of about 9 K/W can be used as a thermalbridge element. This relatively low value of R_(TB) can be of the sameorder as the value of R_(OB) (6-7 W/mK). In order to create a heat flowpath from the gain chip to the portion of the package at theenvironmental temperature through the thermal bridge elementcorresponding to a higher effective thermal resistance, i.e., at least1.5 larger than R_(OB), the copper wire is placed at a relatively largedistance from the gain chip so that the thermal resistance of the heatflow path from the end of the copper wire to the gain chip is notnegligible with respect to the value of R. For instance, a first end ofthe copper wire is glued by means of Ag-filled epoxy on the bonding padof a package feed-through and a second end of the copper wire is gluedon a vertical wall (vertical with respect to the optical bench) of thesubmount of the gain chip. For example, the thermal resistance of theheat flow path between the package portion at T_(env) and the gain chipis of about 11 K/W.

FIG. 14 shows a cross-sectional perspective view of a portion of a lasermodule 20′, according to a preferred embodiment of the invention. Thethermal bridge element 81 is described more in detail with reference toFIG. 13. The laser cavity comprises a gain chip 82, a collimating lens89, an etalon and a tuneable mirror. The etalon and the tuneable mirrorare not shown in FIG. 14, which shows only the mounting structure 89housing the etalon. The gain chip 82 is a chip-on-submount (COS) lasersub-assembly, including a semiconductor laser chip mounted of a smallthermally conductive submount, e.g., 2×1 mm². The COS laser 82 ismounted on a first thermally conductive submount 83 made for instance ofSiC. The first submount 83 is mounted on a second thermally conductivesubmount 84 made for instance of CuW 80/20 or of AlN. Submount 84 isbonded on a thermally conductive platform 90, which mounts the laserassembly. The thermally conductive platform 90, i.e., the optical bench,is placed on a TEC (not shown). Collimating lens 89 is housed in amounting structure 88, e.g., a metallic structure such as in Kovar,which is laser welded on the optical bench 90. A mounting structure 88houses a FP etalon 92 (only a small portion of the etalon is shown inFIG. 14) can be for instance soldered on the optical bed. If the opticalbench is of ceramic material, the portion on which the mountingstructure 88 and/or 89 is laser welded or soldered is metallised.

The laser assembly of FIG. 14 is housed in a package defining anenclosure comprising lateral walls, a base and at least a feed-throughstructure. Only a cross-sectional view of a lateral wall 87 and aportion of a feed-trough 85 are illustrated in FIG. 14. The uppersurface of the package feed-through 85, i.e., the surface facing thegain chip 82, includes a plurality of bonding pads 86 extending bothinternally and externally the package enclosure (as for instance instandard ceramic butterfly packages). On the external side of thefeed-through, a plurality of leads 91 are soldered on the bonding pads86.

Thermal bridge element 81 thermally couples the gain chip 82 to thepackage feed-through 85. The thermal bridge element is soldered on oneend to the submount 83 in the proximity of the COS 82 and on the otherend to a bonding pad 86. For instance, an end portion of the thermalbridge element is soldered at 1 mm from the edge of the COS 82.

1. An external-cavity laser module comprising: a package defining anenclosure, said package comprising a base having a surface internal tothe enclosure; a thermoelectric cooler within said enclosure, saidthermoelectric cooler comprising an upper carrier plate and a lowercarrier plate, said lower carrier plate being in thermal contact withsaid internal surface of said base and said thermoelectric cooler beingconfigured to stabilise the temperature of the upper carrier plate; alaser assembly, housed within said enclosure, comprising a gain mediumfor emitting an optical beam into an external cavity, said gain mediumbeing thermally coupled to the upper carrier plate of the thermoelectriccooler via at least one thermally conductive element so as to provide afirst heat flow path having a first thermal resistance; and a thermalbridge element having first and second arms disposed on opposite ends ofan articulated resilient central portion, the first arm placed inthermal contact with the gain medium and the second arm placed inthermal contact with the package, the thermal bridge element providing asecond heat flow path having a second thermal resistance between saidgain medium and said package, wherein the second thermal resistance isgreater than the first thermal resistance, the second thermal resistancebeing selected to at least partially compensate for thermally inducedvariations of an optical path length of the external cavity, so as tomaintain a substantially stable laser output power; and wherein thefirst thermal resistance is greater than about 5 K/W.
 2. The lasermodule of claim 1, wherein said first thermal resistance is less thanabout 10 K/W.
 3. The laser module of claim 2, wherein said first thermalresistance is between about 5 K/W and 8 K/W.
 4. The laser module ofclaim 1, wherein said thermoelectric cooler comprises a Peltier cellsandwiched between said upper carrier plate and said lower carrier plateof the thermoelectric cooler.
 5. The laser module of claim 1, whereinthe resilient central portion comprises two straight portions abutted soas to form a V-shape longitudinal cross-section and said two arms aretwo straight portions substantially parallel to one another.
 6. Thelaser module of claim 5, wherein said thermal bridge element furthercomprises grooves formed at the hinges between each pair of straightportions.
 7. The laser module of claim 1, wherein said enclosurecomprises a lateral wall and said package further comprises afeed-through structure providing an electrical interconnect into or outof said lateral wall of said enclosure, and said second end of saidthermal bridge element is placed on said feed-through structure.
 8. Thelaser module of claim 1, further comprising a thermally conductiveplatform in thermal contact with the upper carrier plate of thethermoelectric cooler, said gain medium being placed on said thermallyconductive platform.
 9. The laser module of claim 8, wherein said laserassembly further comprises an end mirror.
 10. The laser module of claim8, wherein said end mirror is in thermal contact with said thermallyconductive platform.
 11. The laser module of claim 10, wherein said endmirror is placed on said thermally conductive platform so as to providea heat flow path to the upper carrier plate of the thermoelectric coolerhaving a third thermal resistance less than said second thermalresistance.
 12. The laser module of claim 11, wherein said third thermalresistance is not greater than 2 K/W.
 13. The laser module of claim 8,wherein: said upper carrier plate of the thermoelectric cooler has afirst length along a main longitudinal direction; said thermallyconductive platform has a second length along said main longitudinaldirection, said second length being greater than said first length; andsaid gain medium is placed on said thermally conductive platform at agiven distance along said longitudinal direction from said upper carrierplate.
 14. The laser module of claim 13, wherein said distance is notless than 2 mm.
 15. The laser module of claim 14, wherein said distanceis between 3 and 5 mm.
 16. The laser module of claim 8, wherein thethermally conductive platform has a thermal conductivity not less thanabout 120 W/mK.
 17. The laser module of claim 8, further comprising athermally conductive submount upon which said gain medium is placed,said submount being placed on said thermally conductive platform. 18.The laser module of claim 17, wherein said thermally conductive submounthas a thermal conductivity not less than about 120 W/mK.
 19. The lasermodule of claim 1, wherein said gain medium is a semiconductor laserchip.
 20. The laser module of claim 1, wherein said laser assembly isconfigured to emit output radiation and said external cavity defines aplurality of cavity modes, said laser module further comprising: a gridgenerator arranged in the external cavity to define a plurality of passbands substantially aligned with corresponding channels of a selectedwavelength grid; and a tuneable element arranged in the external cavityto tuneably select one of said pass bands so as to select a channel towhich to tune the optical beam.
 21. The laser module of claim 20,wherein said tuneable element is a tuneable mirror functioning also asend mirror of the external cavity.
 22. The laser module of 20, whereinthe grid generator is arranged in the laser cavity in the optical pathof the laser beam emitted by the gain medium between the gain medium andtuneable mirror.
 23. The laser module of claim 20, wherein the gridgenerator is a Fabry-Perot etalon.
 24. The laser module of claim 20,further comprising a thermally conductive platform, wherein said gainmedium is in thermal contact with said thermally conductive platform.25. The laser module of claim 24, wherein said grid generator and saidtuneable element are in thermal contact with said thermally conductiveplatform.
 26. The laser module of claim 20, wherein the laser assemblyis configured to emit output radiation at a laser emission frequency ona single longitudinal mode.
 27. The laser module of claim 1, wherein thesecond thermal resistance is selected to at least partially compensatefor thermally-induced variations of an optical path length of theexternal cavity, so as to maintain a substantially stable laser outputpower.
 28. The laser module of claim 27, wherein the laser output powerfluctuates by less than 1 dB.
 29. An external-cavity laser modulecomprising: a package defining an enclosure, said package comprising abase having a surface internal to the enclosure; a thermoelectric coolerwithin said enclosure, said thermoelectric cooler comprising an uppercarrier plate and a lower carrier plate, said lower carrier plate beingin thermal contact with said internal surface of said base and saidthermoelectric cooler being configured to stabilise the temperature ofthe upper carrier plate; a laser assembly, housed within said enclosure,comprising a gain medium for emitting an optical beam into an externalcavity, said gain medium being thermally coupled to the upper carrierplate of the thermoelectric cooler via at least one thermally conductiveelement so as to provide a first heat flow path having a first thermalresistance; and a thermal bridge element having first and second armsdisposed on opposite ends of an articulated resilient central portion,the first arm placed in thermal contact with the gain medium and thesecond arm placed in thermal contact with the package, the thermalbridge element providing a second heat flow path having a second thermalresistance between said gain medium and said package, wherein saidsecond thermal resistance is greater than said first thermal resistanceby a factor of between about 1.5 and about 12; and wherein the firstthermal resistance is greater than about 5 K/W.
 30. The laser module ofclaim 29, wherein said second thermal resistance is greater than saidfirst thermal resistance by a factor of between about 2 and about 12.31. The laser module of claim 30, wherein said second thermal resistanceis greater than said first thermal resistance by a factor between about5 and 12.